The present invention relates to an inverter apparatus using a microcomputer control system, and more particularly, to an inverter apparatus suitable for use in driving an ac rotary electric machine such as an induction motor, a rotary machine including a synchronous machine and a dc machine.
An inverter apparatus is widely used in variable speed control of an induction motor. Recently, inverters controlled by a microcomputer are being utilized and various control functions of the inverters can be attained by the microcomputer.
However, in a system using such an inverter apparatus, if an output frequency of the inverter apparatus is increased greatly due to any trouble such as, for example, external noise, a rotational speed of the induction motor is also increased and in a large system, particularly, there is a fear that any instrument may be destroyed.
In such a system, heretofore, it is known that a predetermined upper limit for the output frequency of the inverter is set as described in Japanese Patent Unexamined Publication No. 62-201081.
A parameter setting device of a motor controller described in the Publication 62-201081 includes memory holding function provided in parameter adjusting means as parameter storing means independently. More particularly, a plurality of parameter adjusting means are replaced by a combination of a single parameter adjusting means, parameter selecting means for specifying a kind of parameter and parameter storing means for storing parameter values in predetermined locations.
In the above prior art as disclosed in the Publication 62-201081, the setting function of the predetermined upper limit is attained by a software processing of the microcomputer. More particularly, in the prior art, the predetermined upper limit of frequency is set as a memory data and the upper limit is compared in the software manner with a binarized signal for controlling an output frequency supplied to a main circuit of the inverter by the microcomputer to control the output frequency of the inverter not to exceed the upper limit.
In the prior art, since the upper limit of frequency set as the memory data is generally converted into a binary code, the upper limit is easily re-written into another value by noise and the whole system is stopped. Further, confirmation as to whether the frequency control signal produced by the microcomputer is correct or not is not taken into consideration. Thus, there is a problem that the upper limit for the output frequency of the inverter is not set reliably.